Process for sintering cubic system silicon carbide powder

ABSTRACT

A process for sintering cubic system silicon carbide powder, which comprises compacting a mixture of cubic system silicon carbide powder with more than 1% by weight and not more than 3% by weight of carbon and at least 0.10% by weight and less than 0.3% by weight of boron and sintering the compact thereby obtained, under vacuum or in a chemically inert atmosphere at a temperature of from 1,900° to 2,200° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for sintering a powder ofcubic system silicon carbide (hereinafter referred to as "β-SiC").

2. Description of the Prior Art

Sintered products of silicon carbide are widely used as abrasionresistant and heat resistant materials by virtue of their superiorhardness and high-temperature strength. The silicon carbide is availablein two crystal forms, i.e. α- and β-forms. The present invention isconcerned with the process for sintering β-SiC powder.

As a process for sintering β-SiC powder, it has been known to mix thepowder with predetermined amounts of boron and carbon, and sinter themixture in a N₂ gas atmosphere or in an inert gas atmosphere.

In the sintering of β-SiC powder, the difficulty in obtaining a highdensity product is attributable to the fact that grain growth takesplace at the final stage of the sintering and coarse grains having agrain size of e.g. 100 μm or more will thereby be formed in asubstantial amount, whereby a high density is hardly attainable. Boronis effective for an increase of the density of the sintered body, but atthe same time, it has a function to facilitate the grain growth at thefinal stage of the sintering. Carbon serves effectively for the removalof SiO₂ (which hinders the sintering) contained as an impurity in theβ-SiC powder, but it has been believed that carbon is detrimental ifused in excess of the amount required for the deoxidation.

Thus, the amounts of boron and carbon to be mixed with the β-SiC powderused to be restricted to certain specific ranges. For instance, it hasbeen proposed to use from 0.5 to 5.0% by weight of boron and from 1.5 to5.0% by weight of carbon (Japanese Examined Patent Publication No.17146/1983), or from 0.3 to 3% by weight of boron and from 0.1 to 1.0%by weight of carbon (Japanese Examined Patent Publication No.32035/1982). Namely, it has been generally believed that it is necessaryto incorporate boron in an amount of at least 0.3% by weight and not toincorporate carbon excessively.

However, these conventional processes still had a difficulty that it wasthereby impossible to adequately control the abnormal grain growth andto obtain a sintered body having an adequately high density.

Further, Japanese Examined Patent Publication No. 46996/1980 proposes aprocess comprising mixing from 0.1 to 5% by weight of each of carbon andboron to a β-SiC powder prepared by a certain specific method andsintering the mixture. According to this process, it is allegedlypossible to use boron in an amount as small as 0.1% by weight. However,in all the Examples given in this publication, boron is used in anamount greater than 1.0% by weight, and the use of boron in a smalleramount is not substantiated. Besides, this process has a seriousdrawback that the β-SiC powder must be prepared by a complicated method.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above-mentioneddifficulties inherent to the conventional processes and to provide aprocess for sintering β-SiC powder, whereby a high density sinteredproduct is readily obtainable without leading to substantial graingrowth.

As a result of extensive researches, the present inventors have foundthat the grain-boundary energy of β-SiC particles can be lowered byinterposing amorphous carbon (or other amorphous inorganic substancessuch as glass) at the grain boundary and that the sintering of the β-SiCpowder will be facilitated and the grain growth will be prevented orsuppressed by the presence of the carbon at the grain boundary, wherebya high density sintered product is obtainable by using a relativelysmall amount of boron. On the basis of this discovery, the presentinvention provides a process for sintering cubic system silicon carbidepowder, which comprises compacting a mixture of cubic system siliconcarbide powder with more than 1% by weight and not more than 3% byweight of carbon and at least 0.10% by weight and less than 0.3% byweight of boron and sintering the compact thereby obtained, under vacuumor in a chemically inert atmosphere at a temperature of from 1,900° to2,200° C.

It has also been found that this process can be further improved bysubjecting the compact to pretreatment which comprises heating thecompact at a temperature of from 1,100° to 1,500° C. under reducedpressure of from 10⁻¹ to 10⁻³ atm, and removing CO gas therebygenerated, prior to the sintering. Namely, when no such pretreatment ofthe compact is conducted, the amount of boron to be incorporated intothe β-SiC powder is preferably at least 0.15% to obtain a high densityproduct. Whereas, when the pretreatment is conducted, the amount ofboron may be reduced to a level of 0.10% by weight.

Now, the present invention will be described in detail with reference tothe preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a graph illustrating the relation between the carbon contentsand the densities of β-SiC sintered products prepared from mixturescontaining various carbon sources.

FIG. 2 is a graph illustrating the relation between the boron contentsand the densities of the sintered products.

FIG. 3 is a graph illustrating the boron contents and the grain growthat the final stage of the sintering.

FIG. 4a is a scanning electron microscopic photograph of a fracturedsurface of the sintered product obtained in Example 1.

FIG. 4b is a scanning electron microscopic photograph of an etchedsurface of the same sintered product.

FIGS. 5a and 5b are scanning electron microscopic photographs of thesintered products of Comparative Samples A and B, respectively.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the amount of carbon to be incorporated into theβ-SiC powder is desirably within a range of more than 1% by weight andnot more than 3% by weight, although it may vary more or less dependingupon the particular carbon source used. If the amount of carbon is notmore than 1% by weight, the thickness of the grain-boundary phaseinterposed at the grain boundary will be inadequate, whereby it will bedifficult to obtain a high density sintered product. On the other hand,if the amount exceeds 3% by weight, the excess amount adversely affectsthe improvement of the density.

In the present invention, the carbon is incorporated into the β-SiCpowder preferably by a method which comprises uniformly applying asolution of a carbon source polymer such as pitch tar, a furan resin, aphenol resin, a polyimide resin or a polyacrylonitrile resin in asolvent such as acetone, benzene or an alcohol, to the surface of theβ-SiC powder, drying the powder and gradually heating the dried powderto a temperature of 400° to 550° C. at a rate of e.g. 30° C./hr in achemically inert atmosphere to form carbon on the surface of the powder.

FIG. 3 illustrates the relation between the boron content and thedensity and grain growth (the sintering temperature: 2,050° C.) whereinthe black circle indicates the grain growth, the white circle indicatesno grain growth and the numerical values indicate the densities. Asshown in the Figure, the optimum amount of boron to be incorporated intothe β-SiC powder is within a range of at least 0.10% by weight and lessthan 0.3% by weight. Namely, if the amount of boron is 0.3% by weight orgreater, the grain growth takes place, and if the amount is less than0.10% by weight, the density of the sintered product tends to beinadequate.

The boron is incorporated in the form of metallic boron, boron carbideor a boron compound which is convertible to boron when heated.

The mixture of β-SiC powder with carbon and boron, is then compacted,and the compact thereby obtained is sintered under vacuum or in achemically inert atmosphere such as an argon atmosphere, at atemperature of from 1,900° to 2,200° C. If the temperature is lower than1,900° C., no adequate sintering can be accomplished, whereby it isdifficult to obtain a sintered product having an adequately highdensity. On the other hand, a temperature higher than 2,200° C. is notrequired for the sintering, and such a high temperature is uneconomical.

According to another aspect of the present invention, the sinteringprocess can further be improved by subjecting the compact topretreatment prior to the sintering. The pretreatment comprises heatingthe compact at a temperature of from 1,100° to 1,500° C. under reducedpressure of from 10⁻¹ to 10⁻³ atm, and removing CO gas therebygenerated. This is based on the following reaction system and discovery.

Namely, SiO₂ contained in the β-SiC powder reacts with the added carbonat a temperature of at least 1,000° C. to generate CO gas, in accordancewith the following reaction (1) or (2):

    SiO.sub.2 +C=SiO+CO                                        (1)

    SiO.sub.2 +3C=SiC+2CO                                      (2)

Then, the CO gas will react with the added boron to form B₂ O₃ gas, inaccordance with the following reaction:

    3CO+2B=B.sub.2 O.sub.3 +3C                                 (3)

Thus, the added boron is consumed by the reaction (3). Therefore, ifSiO₂ is removed prior to the sintering so that the consumption of boronby the reaction (3) as avoided the amount of boron required can bereduced accordingly. For this purpose, the compact is treated underreduced pressure of from 10⁻¹ to 10⁻³ atm at a temperature of from1,100° to 1,500° C. prior to the sintering. Namely, the reactions (1)and (2) take place at 1,100° to 1,500° C. under 10⁻¹ to 10⁻³ atm, andthe generated CO gas is discharged from the system without being reactedwith boron, whereby the reaction (3) is avoided and no consumption ofboron takes place.

When no such pretreatment of the compact is conducted, it is preferredto incorporate boron in an amount of at least 0.15% by weight relativeto the β-SiC powder to obtain a sintered product having an adequatelyhigh density. This minimum amount of boron can be further reduced to alevel of 0.10% by weight by conducting the pretreatment. FIG. 2 showsthat when the pretreatment is conducted (i.e. the curve 2), the amountof boron may be from 0.10 to 0.3% by weight and a density as high as3.18 g/cm³ is thereby obtainable, whereas when no such pretreatment isconducted (i.e. the curve 1), the minimum amount of boron required, issubstantially higher because boron is consumed by the above-mentionedreaction (3) and yet the density is lower than that obtainable with thepretreatment.

It has been known that it is desirable to remove SiO₂ from the β-SiCpowder prior to the sintering (Japanese Unexamined Patent PublicationNos. 166369/1982 and 166372/1982). However, in each of these prior artprocesses, the powder is treated in a reducing gas (H₂ or CO) atmosphereunder reduced pressure to activate the powder surface, whereby SiO₂ isremoved by the following reactions:

    SiO.sub.2 +H.sub.2 =SiO+H.sub.2 O

    SiO.sub.2 +CO=SiO+CO.sub.2

It is also disclosed that in a case where no reducing gas is employed,it is advisable to use a high vacuum condition of from 10⁻⁴ to 10⁻⁷ atm,whereby the following reaction is conducted.

    SiO.sub.2 =SiO+1/2O.sub.2

These processes where the reducing gas is employed, are not onlydangerous but also cumbersome in their operation. On the other hand, inthe case where a high vacuum condition is employed, an expensivesophisticated exhaust gas discharge system is required to maintain thevacuum condition at a level of from 10⁻⁴ to 10⁻⁷ atm in an ordinarysintering furnace.

Whereas, according to the present invention, the compact is heated at atemperature of from 1,100° to 1,500° C. under a relatively low vacuumcondition at a level of from 10⁻¹ to 10⁻³ atm whereby the CO gasgenerated may readily be removed. Thus, the process of the presentinvention does not require the use of any reducing gas, and the CO gascan be removed by a simple exhaust gas discharge system. The generatedCO gas immediately be discharged at a temperature of not higher than1,500° C. under reduced pressure, whereby it will not be consumed by thereaction with boron.

If the temperature is lower than 1,100° C., SiO₂ is hardly decomposed.On the other hand, if the temperature is higher than 1,500° C., SiCtends to decompose and the sintering will thereby be adversely affected.Thus, the temperature should be from 1,100° to 1,500° C. If the pressureis higher than 10⁻¹ atm, the decomposition to CO gas and the dischargethereof tend to be slow. On the other hand, it is unnecessary to reducethe pressure beyond 10⁻³ atm and such an excessively high vacuumcondition is uneconomical.

Now, the present invention will be described in further detail withreference to Example and Comparative Examples.

EXAMPLE 1

β-SiC powder identified in Table 1 was used. The β-SiC powder wasprepared by reacting SiO₂ with carbon.

                  TABLE 1                                                         ______________________________________                                        β-SiC powder used as starting material                                   True specific gravity                                                                           3.19 to 3.22 g/cm.sup.3                                     Crystal form      Cubic system crystal (3C)                                   Average particle size                                                                           0.25 to 0.28 μm                                          Particles having a                                                                              95 to 98%                                                   size of not larger                                                            than 1 μm                                                                  Specific surface area                                                                           15.1 to 18.7 m.sup.2 /g                                     Impurities        Al 0.03 to 0.06%                                                              Fe 0.03 to 0.07%                                                              SiO.sub.2 0.22 to 0.33%                                                       C 0.34 to 0.47%                                             ______________________________________                                    

As the carbon source, a phenol resin was used. The phenol resin wasdissolved in acetone. The solution thereby obtained, was applied to theβ-SiC powder in such an amount that the remaining carbon constitutes 2%by weight. Then, the powder was adequately dried in a vacuum drier atabout 100° C. for 24 hours, and gradually heated to a temperature offrom 400° to 550° C. in a furnace of an argon gas atmosphere at atemperature raising rate of 3° C./min, whereby carbon was uniformlyformed on the surface of the β-SiC powder. Then, 0.2% by weight of boronwas added thereto. The mixture was thoroughly mixed and compacted by arubber press to form a compact having a density of about 60% based onthe theoretical density.

The compact was heated to 1,400° C. and vacuumed to a level of 10⁻³ atm,whereby the generated CO gas was discharged.

The compact thereby obtained, was sintered in an argon gas atmosphere of1 atm at about 2,100° C. for 15 minutes.

The sintered product was composed of grains having a size of about 3 toabout 10 μm, and no abnormal grain growth was observed. The density wasas high as 3.15 g/cm³ (i.e. 98% of the theoretical density). Thus, theproduct was a sintered body having a high density and no abnormal graingrowth.

The characteristics of the sintered product are given in Table 2, andthe SEM photographs of the its structure are shown in FIGS. 4a and 4b.FIG. 4a shows the fractured surface and FIG. 4b shows an etched surfaceof the sintered product.

                  TABLE 2                                                         ______________________________________                                        Characteristics of β-SiC sintered product                                Amount of added carbon                                                                          2% by weight                                                Amount of added boron                                                                           0.2% by weight                                              Density           3.15 g/cm.sup.3                                             Grain size        3 to 10 μm                                               Strength          At room temperature: 600 MPa                                                  At 1,500° C.: 650 MPa                                ______________________________________                                    

COMPARATIVE EXAMPLE 1

Comparative Samples A and B were prepared in the same manner as inExample 1. In Sample A, boron was added in an amount of 0.4% by weight,i.e. an excess amount. Whereas in Sample B, carbon was added in anamount of 0.25% by weight, i.e. less than 1/4 of the optimum amountaccording to the present invention. The characteristics of the sinteredproducts and the SEM photographs of their structures are shown in Table3 and FIGS. 5a and 5b, respectively.

                  TABLE 3                                                         ______________________________________                                        Comparative Examples                                                                          Sample A   Sample B                                           ______________________________________                                        Amount of added carbon*                                                                         1.7%         0.5%                                           Amount of added boron                                                                           0.4%         0.25%                                          Density           3.16 g/cm.sup.3                                                                            2.34 g/cm.sup.3                                Structure         Abnormal grain                                                                             Insufficient                                                     growth       density                                        ______________________________________                                         *The carbon source of Sample A was pitch tar.                            

In the case of Sample A, boron was added in an excess amount, andaccordingly, abnormal grain growth took place to a great extent (seeFIG. 5a) although an adequate density was obtained. Thus, Sample A isnot qualified as a high performance material. On the other hand, in thecase of Sample B, the amount of added carbon was insufficient, and noadequate density was obtained and the product was a porous and brittlematerial.

Further, it should be apparent from FIGS. 1 and 3 that no adequatedensity is obtainable when the amount of added boron is insufficient(less than 0.10%) or when the amount of added carbon is excessive (3% ormore).

From the comparison of the Example with the Comparative Examples, it isevident that according to the present invention, a high density isattainable with a small amount of boron and a sintered product having ahigher density and less grain growth than the conventional products, isobtainable by adding an adequate amount of carbon to form a secondarygrain-boundary phase (i.e. from 1 to 3% by weight).

What is claimed is:
 1. A process for sintering a cubic system siliconcarbide powder, which comprises:(i) compacting a mixture of cubic systemsilicon powder with more than 1% by weight and not more than 3% byweight of carbon, and at least 0.10% by weight and less than 0.3% byweight of boron; (ii) subjecting the said compact to pretreatment whichcomprises heating the compact at a temperature of from 1100° C. to 1500°C. under a reduced pressure of from 10⁻¹ to 10⁻³ atm, and removing COgas thereby generated; and, (iii) sintering the said compact obtained,under vacuum or in a chemically inert atmosphere at a temperature offrom 1900° C. to 2200° C.
 2. The process according to claim 1, whereinthe mixture contains at least 0.15% by weight and less than 0.3% byweight of boron.
 3. The process according to claim 1, wherein the carbonis incorporated into the mixture by uniformly applying a solution of acarbon source polymer to the surface of the cubic system silicon carbidepowder, drying the powder and gradually heating the dried powder to atemperature of 400° to 550° C. in a chemically inert atmosphere to formcarbon on the surface of the powder.
 4. The process according to claim3, wherein the carbon source polymer solution is a solution of a carbonsource polymer selected from a group consisting of pitch tar, a furanresin, a phenol resin, a polyimide resin and a polyacrylonitrile resin,in a solvent selected from a group consisting of acetone, benzene and analcohol.
 5. The process according to claim 1, wherein the boron isincorporated into the mixture in a form of metallic boron, boron carbideor a boron compound convertible to boron when heated.
 6. The processaccording to claim 1, wherein the chemically inert atmosphere is anargon gas atmosphere.